Makefile: move fs/common.mk above external.mk

The file external.mk was included before fs/common.mk, so it was impossible
to add rootfs targets using the BR2_EXTERNAL mechanism.
This change moves the inclusion of fs/common.mk before external.mk to allow
this.

Signed-off-by: Philippe Reynes <tremyfr@yahoo.fr>
[ThomasDS: rebased, update commit message]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
diff --git a/Makefile b/Makefile
index d91231c..4b93805 100644
--- a/Makefile
+++ b/Makefile
@@ -387,6 +387,7 @@
 include boot/common.mk
 include linux/linux.mk
 include system/system.mk
+include fs/common.mk
 
 include $(BR2_EXTERNAL)/external.mk
 
@@ -400,8 +401,6 @@
 TARGETS += toolchain-eclipse-register
 endif
 
-include fs/common.mk
-
 TARGETS_SOURCE := $(patsubst %,%-source,$(TARGETS))
 TARGETS_DIRCLEAN := $(patsubst %,%-dirclean,$(TARGETS))